Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic system, such as a laser beam printer, a copying machine, or a facsimile machine.
Description of the Related Art
An image forming apparatus using an electrophotographic system includes a developing device (a developing unit) that allows a developer to adhere to an electrostatic latent image formed on the surface of a photoreceptor (an image bearing member) to develop the electrostatic latent image into a visible image.
Further, the development system of the developing device of the image forming apparatus can be broadly divided into a one-component development system and a two-component development system. Particularly, an image forming apparatus employing a two-component development system, which allows a two-component developer including toner and a carrier to be borne on a rotatable developing sleeve and develops an electrostatic latent image formed on a photoreceptor, is widely used.
In the image forming apparatus employing the two-component development system, only toner included in a developer is consumed with the formation of an image. Accordingly, toner density, which is the ratio of the weight of toner included in the developer, is reduced. For this reason, a toner density sensor, which detects the density of toner included in the developer, is provided and toner is supplied so that toner density is maintained in a predetermined range.
Here, when an image is formed, toner is consumed but a carrier is basically accumulated in the developing device. However, a carrier deteriorates with an increase in the number of sheets on which images are formed, so that the charging performance of the carrier gradually deteriorates. There is a concern that the quality of an image may be caused to deteriorate when the charging performance of the carrier deteriorates.
Accordingly, a structure that suppresses the deterioration of a carrier present in the developing device has been proposed in the past. For example, Japanese Patent Laid-Open No. 2009-300645 discloses a structure that supplies only a carrier or a mixture of a carrier and toner to a developing device and discharges a developer, which is present in the developing device, on one side to exchange the developer. According to this structure, the percentage of the deteriorated carrier included in the developer, which is present in the developing device, is reduced. Accordingly, the deterioration of the quality of an image, which is caused by the deteriorated carrier, can be suppressed.
Here, in the structure disclosed in Japanese Patent Laid-Open No. 2009-300645, a toner density sensor, which detects toner density in the developing device, and a developer amount sensor, which detects the amount of a developer, are provided and the amount of a developer to be supplied to the developing device is determined based on the detection results of these sensors. Since a plurality of sensors is disposed in the developing device as described above and the amount of a developer is detected in addition to toner density, the detection of toner density is hardly affected even though the amount of a developer present in the developing device is changed with, particularly, the supply of a developer.
However, the cost of the developing device is increased in a structure in which sensors are provided in the developing device as in the structure disclosed in Japanese Patent Laid-Open No. 2009-300645. Further, since the control of the toner density sensor needs to be adjusted when the developing device is replaced, a working process needs to be performed at the time of replacement. For this reason, a structure, in which a toner density sensor is disposed in an image forming apparatus even though the developing device is replaced, is preferable. A structure, in which the toner density of a developer discharged from the developing device is detected since the developer discharged from the developing device is relatively similar to a developer present in the developing device, can be employed as a method therefor. However, since the amount of a developer to be discharged is not sufficient, a detection result is likely to be unstable in some cases of the arrangement of the toner density sensor. For this reason, an arrangement in which detection accuracy is improved is desired.